my $repl = "--$1";            ===  my $repl = '--'.$1;

my $repl = shift;             ===  my $repl = '--$1';

my $repl = eval shift;        ===  my $repl = --$1;
                              ===  Modification of a read-only value

my $repl = eval qq{"$temp"};  ===  my $repl = "--$1";
                              ===  my $repl = '--'.$1;

##</code><code>##

s/.../eval qq{"$temp"}/e